AQ 232 – Fish Population Dynamics and Stock Assessment

Introduction

• Estimating fish stock abundance is important for sustainable fisheries management.
• Various methods exist, each with strengths and limitations.
• Choice of method depends on species, habitat, data availability, and resources.

Fish abundance

• Fish abundance refers to the number of individuals in a fish population within a defined area.

• It is a key metric for assessing the health and sustainability of fish stocks.

• Accurate abundance estimates inform management decisions, such as setting catch limits and conservation measures.

• Abundance can be expressed as:

  • Absolute abundance – Measure the actual number of fish in an area i.e Pond
  • Relative abundance – Estimate the number of fish at a given locality

Fish abundance…

• Abundance is not a fixed quantity – Varies from time to time (seasonal) due to;

  • Internal forces i.e births and deaths and
  • External forces i.e seasonal availability of food, changes in environmental conditions that may affect the population

• It varies from place to place because of the heterogeneity of the environment

Fish abundance…

• Within the area of distribution the abundance of a population always vary – either grow (increase) or decline

• The population may vary with season due to;

  • Births
  • Deaths
    
  • Seasonal migration (spawning, feeding and over-wintering) recruitment

Fish migration

• Fish migration is mass relocation by fish from one area or body of water to another.

• Fish migrate;

  • On a regular basis
    
  • On time scales – from daily, seasonal, annually or
    
  • Longer, and over distances – from a few metres to thousands of kilometres.

• Such migrations are usually done for feeding or to reproduce, but in other cases the reasons are unclear.

• Migration affects local abundance estimates and must be considered in survey design.

Fish migration…

flowchart LR
  A(Spawning </br> Ground) --> B(Feeding </br> Ground)
  B --> C(Wintering </br> Ground)
  C --> A
  B --> A
  A --> C
  C --> B

Terminologies in fish migration

• Diadromous – Fishes that migrate between the sea and freshwater.
• Anadromous – Diadromous fishes which spend most of their lives in the sea and migrate to freshwater to breed (salmon, sea trout, shad, sea lampreys, sturgeons)
• Catadromous – Diadromous fishes which spend most of their lives in freshwater and migrate to the sea to breed (eel, Salangidae, Galaxidae, Retropinnidae).
• Amphidromous – Diadromous fishes which migrate from the sea to freshwater or vice versa, but not for the purpose of breeding (some Exocidae,Perca fluviatilis, some Mugilidae).

Terminologies in fish migration…

• Potamodromous – Migratory fishes that live and migrate wholly within freshwater systems (rivers, lakes, streams). They never enter marine environments e.g (trout, bream, Coregonoids)

  • Complete their entire life cycle in freshwater.
  • Migrate between different freshwater habitats for spawning, feeding, or overwintering

• Their migration patterns are triggered by Water temperature changes, Flooding (access to floodplain habitats), Photoperiod (day length), Food availability, Spawning readiness

Terminologies in fish migration…

• Oceanodromous – Migratory fishes that live and migrate wholly within the sea (marine environment). They never enter freshwater. (cod, herring, capelin, tuna, mackerel).
• Complete their entire life cycle in marine waters
• Migrate between different ocean habitats for spawning, feeding, or seasonal movements
• Migration distances can range from coastal movements to trans-oceanic journeys (thousands of kilometers)
• Their migration patterns are triggered by Sea surface temperature gradients, Ocean current patterns (monsoons in Indian Ocean), Prey availability (following zooplankton blooms), Spawning readiness and lunar cycles, Upwelling events

Methods for estimating fish abundance

1. Direct Methods (Surveys)

• Trawl surveys, acoustic surveys, visual surveys
• Provide absolute or relative abundance estimates
• Require dedicated research vessels and equipment

2. Indirect Methods (Fishery-Dependent)

• CPUE, mark-recapture, depletion methods
• Use commercial fishery data
• Cost-effective but subject to biases

3. Statistical Models

• Virtual Population Analysis, integrated models
• Combine multiple data sources
• Require specialized software and expertise

1. Trawl Surveys–Swept area method

• It is assumed that the area swept by the trawl is representative for the entire area over which the fishes are distributed.
• That fish caught within the area swept by the trawl net is proportion to their abundance

Trawl net

Types of Trawl Nets

• Trawl nets that are dragged on the sea floor are called bottom trawls.
• Bottom trawl - Demersal species (snappers, groupers, flatfish)
• Mid-water trawl - Pelagic species (sardines, herring)

Swept area method

Principle of swept area method:

• The mean catch (either in weight or in numbers) per unit of effort or per unit of area is an index of the stock abundance.
• This index is converted into an absolute measure of biomass.

Swept area method…

\[\frac{a}{A} = \frac{c}{B}\]

• The biomass (B) is converted to absolute biomass by multiplying with constant R

\[\frac{a}{A}=\frac{c}{BR}\]

Swept area method…

\[B = \frac{cA}{aR}\]

Where;

B = Biomass a = area swept by the net c = catch A = area of distribution of the fish stock R = retention rate of the net (0.5 - 0.8)

Swept area method…

• A trawl net is used to estimate the mean catch at a number of stations in a fish stock
• A towed trawl net samples fish in an area (a) prescribed by the net’s effective width and distance traveled by the net

Calculations on swept area method

• Let

  a be the area prescribed by the net h is the length of the net head rope

  If the effective width of the net (w) is proportion to the head rope (e.g 0.6);

  Then,

  \(w = 0.6h\)

Swept area method…

• If the towing speed (v) and time for trawling (t) is known then, it is possible to calculate the distance (d) covered during towing

\[d = v\times t\]

• Then, the area prescribed by the net during towing is \[a = w \times d\] \[a = w\times v\times t\]

Question 1

After 0.5 hour of trawling at a speed of 2 km per hour, 3 tons of fish were caught by a trawl net with a head rope measuring 20m. If the retention rate of the net was 50% and only 0.6 of the head rope was making an effective width of the path, calculate the biomass of a fish stock of Solea solea distributed over an area of 4 km²

Solution

\[\frac{a}{A} = \frac{c}{BR}\] \[B = \frac{cA}{aR}\]

Solution

t = 0.5hr

v = 2km/hr

c = 3tons

h = 20m = 0.02km

R = 50% = 0.5

w = 0.6h = 0.6*0.02

A = 4 km2

B = ?

Solution…

\[B = \frac{3tons\times 4km2}{0.6\times0.02km\times2km/hr\times0.5hr\times0.5}\]

\[B = 2000tons\]

Last updated: January 2026